I'm not sure why you are trying to make it hard than it is. If you have 4 servos that are in use at one time, that draw an amp each, you need 4 amps, period. BECs are convenience components, that are used because R/C servos used to burn out if provided with more than 5 volts. 6 Nicad batteries, which was the most common configuration before LiPo technology came around, worked out to 9.6 volts (6 X 1.2v). BECs are available in many sizes, and greater than 3.5 amps is common.
If the requirement is greater than what is available, you simply provide your own voltage/amperage to the servos - often straight from a 6v battery, again the BEC is/was just a convenience component. If you do this with excessive current (more than the controller can pass through) you need to remove the red wire from the plug that goes into the controller, and supply voltage via that red wire. the black wire, needs to be both a ground connection for the signal, so it can not be removed from the controller, but it also needs to be the negative connection for power. The need to do this is pretty uncommon. The controller power connections are just straight pass through.
If you are using a plain R/C receiver the rule of thumb is the circuit board can handle about 6 amps max. A good digital servo will draw 2 amps at full power, but in most cases, what I have seen used in robotics are not good servos, so your guess of about an amp is reasonable.